The present invention pertains generally to electronic test instrumentation, and, more particularly, to a spring and socket assembly for a high bandwidth electronic test probe.
The increasing reliance upon computer systems to collect, process, and analyze data has led to the continuous improvement of the system components and associated hardware. New methods for increasing the speed of integrated circuit components while also increasing the functional density and reducing the physical size of integrated circuits are constantly being sought. As a result, it is not uncommon to see integrated circuits running at several GHz with pin spacing on the order of 10 mil apart.
In a test environment, electronic test instruments such as oscilloscopes and logic analyzers are often required to measure electrical parameters on device pins or nodes of a circuit. A common tool for collecting measurements in this environment is an electrical test probe. An electrical test probe is used to make a connection between a test point or signal source on a device/circuit under test and a test instrument. An electrical test probe comprises a cable having a connector at one end connectable to the electronic instrument and having a contact device such as a probe pin at the other end of the cable for probing the test point (e.g., a desired device pin or circuit node). Typically, the contact device includes a probe pin connected to probe circuitry which filters a signal seen on the probe pin. The probe pin may be manually springably connectable to the probe circuitry via a spring mechanism.
As the speed of integrated circuits increase, the bandwidth required of electrical test probes has exceeded that which can be achieved with prior art probes. As a general rule, in order to achieve accurate measurements, the bandwidth of a test probe should be approximately five times greater than the frequency of the waveform being measured.
FIG. 10 is a top view and FIG. 11 is a cross-sectional side view of a prior art electrical test probe tip 20. As shown, test probe tip 20 includes circuitry implemented on a printed circuit board 22. The printed circuit board 22 includes an input port 23 for receiving signals from a contact spring 25, and an output port 24 for electrical connection to a probe cable 21.
The printed circuit board 22 and probe pin 26 are positioned within a housing 28. In order to achieve maximum electrical contact, prior art contact spring mechanisms 25 were formed as a flat piece of metal with width d shaped into a hook, as illustrated in FIGS. 11 and 12. The width d of such prior art hooks is typically on the order of approximately 100-200 mils wide. Due to the large width d of the contact spring 25, the contact spring 25 exhibits a large parasitic capacitance Chook which prevents signals above a certain cutoff frequency fo from passing. The cutoff frequency of the contact spring 25 is the frequency of the wave when the wavelength λ is twice the width d of the contact spring 25. At this frequency, λ/2 resonances occur that cause the contact spring 25 to act inductively. Above the cutoff frequency, additional resonances occur regularly. Therefore, the cutoff frequency represents the upper limit of the capacitor's (i.e., contact spring 25) frequency range. As is known in the art, the larger the width d of the contact spring, the greater its parasitic capacitance and inductance and therefore the lower the cutoff frequency of the probe.
Accordingly, there exists a need in the industry for a high bandwidth electrical test probe. In particular, a need exists for a probe contact spring of much smaller size and therefore reduced characteristic capacitance that also ensures good electrical contact.
In addition, as the node size and the spacing between nodes is reduced, the size of the probe tips must also accordingly be decreased in order to accommodate the required spacing between the nodes under test. Accordingly, there also exists a need in the industry for an electrical test probe that may be rotated to the desired distance without rotating the entire probe assembly.